Compression Ratio Calculations for Engines

[lkraemer]

Dear Mr. Lee,
I have a Magnum .91 2 Stroke engine with the following Specifications:

Cylinder Bore - 27.70mm
Crankshaft Stroke - 24.80mm
Cylinder Volume Displacement - 14.951cc

Compression Ratio = Unknown???

I assume that the engine as shipped has somewhere around a 9:1 Ratio
with two head shims that equal ~ .010 inches.

What I want to do is know, in detail, is EXACTLY how to measure the HEAD Volume in cc's so I can plug this information into the CR formula and calculate the actual Compression Ratio. I have emailed HobbyPeople for the Compression Ratio, but so far they have not responded to my email.

My goal is to raise the Compression Ratio of that engine to around 13.5:1
and be able to run it on FAI fuel (no Nitro) like the MOKI and MARK engines.
Is this possible, and/or feasible? Will the Rod/Crankshaft withstand the
raise in Compression Ratio from ~9:1 to 13.5:1?

What are your thoughts?

Thanks for your help.

Larry Kraemer

[klickitat]

You can measure your engine with a few tools and a calculater.
First get a medicine dropper from your local pharmacy that measures in cc's.
You will need some thin pencil solder (solid core) and a set of calipers.

#1 pull the plug.
#2 taking a piece of solder and bend it at a 90 degree angle so that the solder when incerted throught the plug hole touches the side of the cylinder.
#3 incert it into the plug hole paralel with the wrist pin and over the top.
#4 now roll the engine over until you smash the solder.
#5 remove solder and measure using your calipers. (this will give you your squish #)
#6 remove cylinder
#8 install plug back into cylinder
#9 using the medicine dropper fill the combustion chamber with water. ( this is the first #) note this number as it is not the complete combustion chamber #. You will also now need to use a calculator to figure the volume of the squish thickness and add to this #. This will give you the total volume of the combustion chamber.
#10 measure from the top of the exhaust port to the top of the side wall of the cylinder and subtract the squish #. Using a calculator you can now figure your compression ratio.

This will give you the true ratio. Many people like to just use the stroke and diameter to combustion chamber volume. thsi is not a true representative because the loss of volume as preasure increases fresh charge is lost out the exhaust until it closes. This will also only work if you are using a muffler and not with a tuned pipe as the pipe will stuff more charge back into the cylinder raising the compression. Another thing to keep in mind is that as the engine warms up the squish will shrink because the rod and piston will grow. Do not forget to leave room or you piston will slap the top of your cylinder.
Compression ratios can be a tricky thing. I much prefer using #'s of preasure. Using a compresion gauge you can get a consistant reading on your engines with out all the math. A little experience will teach you where you need to be with your compression. Start by taking a stock reading from your engines. This will give you a base # to work with and then slowly build up your engines.